The present invention relates to cathode ray tube control circuits. More particularly, it relates to a controlled signal coupling circuit for cathode ray tube apparatus.
In the art relating to cathode ray tube (CRT) display or recording apparatus, a relatively high voltage accelerating potential is required to cause the translation of the electrons from the cathode to the tube face. In conventional CRT display circuits, the cathode of the CRT is held at or near ground and a relatively high positive voltage is applied to a so-called second anode. On the other hand, in CRT recording apparatus in which a fiber optic type target is provided in the faceplate of the CRT, as a safety feature, it is desirable that the structure in and around the front of the tube be at or near ground potential. Accordingly, in such tubes, the cathode is biased at a high negative voltage to provide the accelerating potential for the electrodes. In such tubes, it is common to utilize so-called Z-axis modulation techniques wherein the intelligence signal is applied to a control grid adjacent the cathode. In such circuits, it is necessary that the controlled grid be biased at or near the potential of the cathode. In order to apply a ground-referenced intelligence signal to that control grid, it is necessary to provide a coupling circuit which will absorb the large negative potential between the grid and ground. In previous systems, a gaseous voltage regulator has been used to effect the coupling. Such gaseous regulators, unfortunately, have exhibited the tendency to arc, effectively shorting the voltage regulator and applying the high voltage between the cathode and grid of the CRT. Such high voltage across those elements of the CRT will, of course, destroy the CRT.